Modern cellular communications standards such as EDGE (enhanced Data rates for GSM Evolution), HSDPA (High-Speed Downlink Packet Access), and LTE (Long-Term Evolution) use fast dynamic scheduling of the terminals, controlled by a base station. This allows efficient utilization of the radio resources in the presence of fast varying traffic and radio conditions.
These systems not only schedule different terminals over time, but also allow multiple terminals to be scheduled simultaneously using different radio resources, such as channelization (or spreading) codes or frequency allocations, thereby allowing a very efficient usage of the radio resources.
Fast dynamic scheduling requires scheduling information to be sent from the base station to the terminals. The scheduling information typically includes the identity of the terminal(s) being scheduled and the radio resources allocated to each terminal. The radio resources can for example be time slots, channelization (spreading) codes, frequency allocation, and spatial allocation, depending on the technology. In addition, other transmission-related information such as modulation format, code rate, transmission precoding weights, and rank can be sent along with the scheduling information.
Because of the desire to adapt the transmission to fast changes, the scheduling information need to be transmitted very often. For instance, in the HSDPA standard the scheduling information is sent with a 2 ms period, and in the LTE standard it is sent with a 1 ms period. The scheduling information is in most cases sent separately to each scheduled terminal. For instance, in the HSDPA standard the HS-SCCH channel (High-Speed Shared Control CHannel) is used to send scheduling information to a terminal. In the LTE standard the PDCCH channel (Physical Dedicated Control CHannel) is used for the same purpose. In the literature it has also been proposed to combine the scheduling information into a single larger scheduling message that is sent to all terminals.
The scheduling messages need to be transmitted with high enough robustness so that the scheduled terminal can receive it with sufficient reliability, since a missed scheduling message means that the scheduled radio resources will be wasted. At the same time, it is desired that the transmission of the scheduling message uses as little system resources as possible, not to waste system resources such as bandwidth, transmission power etc. that could otherwise be used for transmission of regular user data. Therefore, the robustness of each scheduling message is in many cases adapted to the radio conditions of the intended terminal. In the case of HSDPA, the transmission power of the HS-SCCH channel can be adapted dynamically based on the expected radio conditions of the intended terminal, thereby optimizing the system resources used for the scheduling information. In LTE, both the transmission power and the code rate of the PDCCH channel can be adapted dynamically for the same purpose.
Nevertheless, transmission of scheduling information generally uses a lot of system resources and it would be desirable to find a way to release at least some of those system resources so that they could be used for transmission of user data.